Fluid Concentration

Electrolytes move between compartments based on the concentration of electrolytes, the gradients of the concentration, and the electrical charge. For example, there is a higher concentration of sodium outside the cell than inside the cell. Therefore, the gradient is towards the inside of the cell.

Fluids move through the body continuously. The heart pumps the blood, pressure is exerted on the vessels from outside the body, and muscles relax and contract to help the heart move the fluid through the vascular system. Fluid moves into and out of the cells and the extracellular spaces by osmotic pressure. This is the pressure exerted by the flow of water through a semipermeable membrane separating two solutions with different concentrations of solute. Osmotic pressure is determined by the concentration of the electrolytes and other solutes in water and is expressed as osmolarity or osmolality. However, the terms are used interchangeably.

Osmolality is the concentration of body fluids. Tonicity is the effect of fluid on cellular volume. Serum osmolality is a better indicator of the concentration of solutes in body fluids than tonicity; tonicity is primarily used as a measurement of the concentration of intravenous solutions.

There are three types of fluid concentration:

• Iso-osmolar. This is a fluid that has the same concentration of particles of solute as water.

• Hypo-osmolar. This is a fluid that has a lower concentration of particles of solute than water.

• Hyper-osmolar. This is a fluid that has a higher concentration of particles of solute than water.

Normal serum has an osmolality of between 275 and 295 mOsm/dg. Less than 275 mOsm/dg is hypo-osmolar and greater than 295 mOsm/dg is hyper-

osmolar. Hypo-osmolar might be caused by a fluid deficit. Hyper-osmolar might be caused by fluid excess.

The concentration of solutes is important when determining the proper replacement fluid for a patient whose fluids and electrolytes are imbalanced. Replacement fluids are replaced orally (by mouth or nasogastric tube) or par-enterally with IV fluids (intravenously or subcutaneously).

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